This invention relates to the spectrophotometric analysis of blood parameters.
Blood parameters or indices such as transvascular fluid exchange, provide important clinical information. For example, there are a number of clinical conditions, e.g. renal failure and massive fluid overload, which require fluid removal and in which it would be beneficial to be able to achieve an optimal rate of fluid removal.
Commonly used clinical approaches for fluid removal include an extracorporeal circuit. Because the body tissues and the blood stream are in direct communication, fluid removal from the blood results in an imbalance of forces which favors fluid movement from the tissues toward the blood stream. Continuous fluid removal from the blood stream therefore results in continuous mobilization from the tissues, which is the ultimate purpose of the therapy.
However, if fluid removal from the blood stream is induced at a rate faster than fluid can be mobilized from the tissues, patients can become hypotensive (shock). This complication is difficult to predict even if blood pressure is followed continuously. To prevent this possibility, fluid removal may be induced at a slow rate. However, it is desirable to achieve a maximal rate of fluid removal which can be matched safely by fluid mobilization from the tissues into the blood stream. In this way, dialysis may be optimized.
As illustrated by U.S. Pat. Nos. 3,830,569 to Meric, 4,243,883 to Schwartzmann, and 4,735,504 to Tycko, a spectrophotometric device for measuring blood indices including a suitable light source and photodetector is known. The light source may be a laser with a collimating lens mounted in front of the laser, and the light source may be remote from or proximate to the blood. As shown by Meric, multiple detectors in combination with a centrally disposed, light trap have been used, and a shield provided with windows may be disposed in the light path. Tycko processes forward scattered light by separating based upon high or low angular interval, prior to detection by separate detectors.
Also known as exemplified by U.S. Pat. No. 4,484,135 to Ishibara et al, is hematocrit determination by blood resistivity measurement. This patent criticizes measurements deriving hematocrit from blood cell count and mean blood cell volume, for using diluted blood if the concentration of the electrolyte and protein in the diluted blood has been changed.
As illustrated by J. Appl. Physiol., 62(1): 364 (1987), light transmissive, plastic tubing through which blood is circulated, may be placed between light sources and light detectors disposed generally opposite from the light sources. As exemplified by J. Appl. Physiol., 69(2): 456 (1990), to ensure detection of a widely scattered light beam, detectors connected in parallel to a photodetector circuit may be used. A separate detector may measure light fluctuations. Diluted perfusate is described.
Unless taken into account, measurement artifacts may induce error in determining blood parameters from optical properties of the blood. One well-known artifact results from changes in the degree of oxygen saturation of hemoglobin (SaO.sub.2). This artifact may be avoided by selecting a specific wavelength or isobestic point at which the hemoglobin oxygen content does not influence light transmission. Isobestic points are known in the IR range (approximately 814 nm), in the green range (approximately 585 and 555 nm), and in the UV range.
As illustrated by U.S. Pat. Nos. 4,745,279 to Karkar et al, 4,776,340 to Moran et al, 5,048,524 to Bailey, 5,066,859 to Karkar et al, and 5,149,503 to Kohno et al, additional light sources or detectors have been used. Karkar '279 uses a detector for diffused light in combination with a compensating detector directly illuminated by another light source. Bailey uses a reference photocell to correct for variation in intensity of a light source, and describes a calibration fluid. Moran uses a catheter including a far detector and a near detector, and a non-linear equation based upon the near/far ratio to calculate hematocrit. Similar to Moran is Kohno, which describes an additional emitter location, and uses a non-linear equation based upon signal ratio to calculate hemoglobin.
Karkar '859 uses a pair of far field detectors and a pair of near field detectors. The far field detectors are spaced equidistant from a light emitting fiber, and one of the far field detectors is used to compensate for the signal detected by its paired fiber. A calibration procedure is described. Hematocrit measurement is compensated for the effects of oxygen saturation, pH and temperature. Karkar '859 points out that neither Karkar '279 nor Moran compensate for the effects of proteins and pH.
Despite the foregoing improvements in using the optical properties of blood to determine blood indices, further improvement is needed to provide a clinically useful methodology and device. Beneficially, monitoring of fluid removal would be made practical.